Higher recording density of magnetic recording media can be achieved by reducing the recording bit length. However, such a reduction in the bit size leads to the problem of data erasure of adjacent tracks in data writing, as well as the problem of S/N degradation of the reproduced signal due to leakage magnetic field from adjacent tracks in data reading. In order to alleviate the influence of adjacent tracks, there have been developed the so-called discrete track media in which a gap is formed each between adjacent tracks, as well as the so-called patterned media in which a gap is formed both between adjacent tracks and between adjacent bits (see U.S. Pat. No. 5,820,769).
An example of the method for inspecting patterned media is a glide test for detecting defects by flying an actual magnetic head over a medium. Another example is an optical test for detecting defects by means of reflected light from a laser.
The glide test uses a dedicated head with a small flying height to scan a medium, causing the head to physically collide with a protrusion defect, and detecting the collision as an electrical signal. This method can detect convex-shaped defects both in data and servo areas, but is not suitable for detecting concave-shaped defects. In addition, the detection time is increased when the entire surface of the medium is scanned by the head.
For the optical test, JP-Publication No. 2002-148207 discloses a method for irradiating a laser beam with a larger spot than the track pitch onto a surface of a medium, and detecting the presence or absence of defects including surface roughness and adhesion of foreign matters, based on the polarization of the reflected light of the laser beam.
The method described in JP-Publication No. 2002-148207 can detect the defect while reducing the influence of the gap formed in the data area of a discrete track medium. However, the laser beam greatly reacts with the servo area pattern physically formed in the discrete track medium as if there were a significant defect, resulting in misinformation. Further, it is difficult to determine the defect based on a certain threshold when the profile shape, which is obtained by the reflected light from the servo area, changes in the radius direction.